The present invention relates to a sealing composition useful for sealing a panel and a funnel of a color cathode ray tube.
A panel and a funnel of a color cathode ray tube used to be sealed by means of a sealing composition containing PbOxe2x80x94ZnOxe2x80x94B2O3xe2x80x94SiO2 type low melting point glass. Namely, such a sealing composition is formed into a paste, which is then coated on the sealing portion and maintained at a temperature of from 440 to 4700xc2x0 C. for from 30 to 40 minutes, whereby the panel and the funnel will be sealed. The interior of the sealed panel and funnel (i.e. a bulb of a color cathode ray tube) will then be evacuated to a high vacuum level of not higher than 10xe2x88x926 Torr at a high temperature of from 300 to 380xc2x0 C.
As such a sealing composition, JP-A-8-225341 discloses a sealing composition comprising PbOxe2x80x94ZnOxe2x80x94B2O3xe2x80x94SiO2 type low melting point glass and 4 wt % of an alumina powder.
In recent years, along with the progress in the trend for large size and flat panel of a color cathode ray tube, the weight of a bulb of a color cathode ray tube has increased (such a weight tends to be further increased, since it is necessary to further increase the glass thickness in order to prevent deterioration of the strength of the bulb of a color cathode ray tube due to the use of a flat panel). Under these circumstances, it is desired to reduce the weight of the bulb of a color cathode ray tube.
In order to reduce the weight of the bulb of a large sized color cathode ray tube, it is necessary to reduce the glass thickness of the panel and the funnel. However, there has been a problem that if the glass thickness is reduced, the strength at the sealing portion of the panel and the funnel decreases.
It is an object of the present invention to solve the above problems and to provide a sealing composition which is capable of improving the strength of the sealed product (such as a bulb of a color cathode ray tube).
The present invention provides a sealing composition having at least one member from xcex1-4PbO.B2O3 crystal powder and Pb3O4 powder incorporated in a total amount of from 0.0001 to 3 parts by weight to 100 parts by weight of a composition comprising at least 80 wt % and less than 98.99 wt % of a powder of PbOxe2x80x94ZnOxe2x80x94B2O3xe2x80x94SiO2 type crystalline low melting point glass, from 0.01 to 5 wt % of a zircon powder, more than 1 wt % and not more than 19.99 wt % of an xcex1-alumina powder and from 0 to 10 wt % of a low expansion ceramic filler.
Now, the present invention will be described in detail with reference to the preferred embodiments.
The sealing composition of the present invention is usually mixed with a vehicle to obtain a paste. Such a vehicle may, for example, be a solution having nitrocellulose dissolved in isoamyl acetate. The paste is coated, for example, at a portion where a panel and a funnel of a color cathode ray tube are to be sealed, and then baked. In such a case, the baking is carried out for example, by maintaining the coated paste at a temperature of from 400 to 450xc2x0 C. for from 30 to 40 minutes.
In the present invention, crystalline glass is a glass which shows a heat generation peak in a differential thermal analysis wherein the temperature is raised at a rate of 10xc2x0 C./min and maintained at from 350 to 500xc2x0 C. for 2 hours. Namely, it is a glass which crystallizes under the above conditions. It is preferably a glass which shows a heat generation peak in a differential thermal analysis wherein the temperature is raised at a rate of 10xc2x0 C./min and maintained at a temperature of from 400 to 450xc2x0 C. for 2 hours.
In the present invention, low melting point glass is a glass having a softening point of not higher than 600xc2x0 C.
In the present invention, PbOxe2x80x94ZnOxe2x80x94B2O3xe2x80x94SiO2 type crystalline low melting point glass is a crystalline glass which contains PbO, ZnO, B2O3 and SiO2 and wherein, when it is maintained at a temperature of from 350 to 500xc2x0 C., first crystals (2PbO.ZnO.B2O3) will appear as the time passes and then second crystals (xcex1-4PbO.B2O3) will precipitate. Preferably, it is a glass, wherein, when maintained at a temperature of from 400 to 450xc2x0 C., the first crystals and the second crystals will precipitate as described above.
In the present invention, a low expansion ceramic filler is a ceramic powder having an average linear expansion coefficient within a range of from room temperature to 300xc2x0 C. (hereinafter referred to simply as an expansion coefficient) being not higher than 70xc3x97107/xc2x0 C. However, zircon powder and xcex1-alumina powder are excluded.
In the sealing composition of the present invention, the low expansion ceramic filler is preferably at least one member selected from the group consisting of cordierite, mullite, lead titanate, silica, xcex2-eucryptite, xcex2-spodumene and xcex2-quartz solid solution, from the viewpoint of handling efficiency.
The interior of the panel and the funnel of a color cathode ray tube, sealed by means of the sealing composition of the present invention, is evacuated usually to a high vacuum level of not higher than 10xe2x88x926 Torr at a high temperature of from 300 to 380xc2x0 C. At that time, the baked sealing composition of the present invention is free from flowing or foaming.
The expansion coefficient of the baked sealing composition of the present invention (hereinafter referred to as the baked product) is preferably from 70xc3x9710xe2x88x927 to 110xc3x9710xe2x88x927/xc2x0 C., more preferably from 80xc3x9710xe2x88x927 to 110xc3x9710xe2x88x927/xc2x0 C., particularly preferably from 85xc3x9710xe2x88x927 to 105xc3x9710xe2x88x927/xc2x0 C., most preferably from 90xc3x9710xe2x88x927 to 100xc3x9710xe2x88x927/xc2x0 C., to adjust it to the expansion coefficients of the panel and the funnel of a color cathode ray tube.
Now, the composition of the sealing composition of the present invention will be described, wherein wt % will be represented simply by %.
The PbOxe2x80x94ZnOxe2x80x94B2O3xe2x80x94SiO2 type crystalline low melting point glass powder is a component which provides fluidity and is essential. If it exceeds 98.99%, the expansion coefficient of the baked product tends to be too large, whereby the sealing portion is likely to break. It is preferably at most 98%. If it is less than 80%, the fluidity tends to be low, and the air tightness of the sealing portion is likely to be low. Preferably, it is at least 85%, more preferably at least 90%.
The zircon powder is a component which accelerates crystallization thereby to shorten the baking time or lowers the expansion coefficient of the baked product to a prescribed level, and thus it is essential. If it exceeds 5%, the fluidity tends to be too low. Preferably, it is at most 1%, more preferably at most 0.5%. If it is less than 0.01%, its effects tend to be too small. Preferably, it is at least 0.05%.
The xcex1-alumina powder is a component which increases the strength and lowers the expansion coefficient of the baked product to a prescribed level, and it is essential. If it exceeds 19.99%, the fluidity tends to be too low. Preferably, it is at most 15%, more preferably at most 10%. If it is less than 1%, its effects tend to be too small. Preferably, it is at least 1.5%, more preferably more than 2%, particularly preferably at least 2.5%, most preferably more than 4%.
The low expansion ceramic filler is a component which lowers the expansion coefficient of the baked product to a prescribed level, and it may be incorporated up to 10%. If it exceeds 10%, the fluidity is likely to be low. More preferably, it is at most 7%, particularly preferably at most 5%, most preferably at most 2%.
The xcex1-4PbO.B2O3 crystalline powder and the Pb3O4 powder are seed crystals for crystallization of the crystalline low melting point glass powder, and they accelerate precipitation of the second crystals (xcex1-4PbO.B2O3). It is essential that at least either component is incorporated. By the precipitation of the second crystals, precipitation of PbO crystals will be suppressed. Precipitation of PbO crystals deteriorates the electrical insulating property of the sealing portion of the panel and the funnel.
Such an electrical insulating property is evaluated by a current (leak current) flowing between the exterior and the interior of the sealing portion when a voltage (30,0000 V) is applied across the exterior and the interior of the sealing portion. The leak current is preferably at most 3 0 nA. He re, the interior of the sealing portion is meant for the surface of the sealing portion on the inner side of the bulb of a color cathode ray tube having the panel and the funnel sealed, and the exterior of the sealing portion is meant for the surface of the sealing portion on the exterior side of the bulb of the color cathode ray tube.
If the total content of the xcex1-4PbO.B2O3 crystal powder and the Pb3O4 powder exceeds 3 parts by weight per 100 parts by weight of the total content of the above four components (i.e. the PbOxe2x80x94ZnOxe2x80x94B2O3xe2x80x94SiO2 type crystalline low melting point glass powder, the zircon powder, the xcex1-alumina powder and the low expansion ceramic filler), the sinterability decreases. The total content is preferably at most 1 part by weight, more preferably at most 0.3 part by weight. If it is less than 0.0001 part by weight, the effect for accelerating precipitation of the second crystals tends to be small. It is preferably at least 0.0002 part by weight, more preferably at least 0.0003 part by weight, particularly preferably at least 0.001 part by weight, most preferably at least 0.01 part by weight.
The xcex1-4PbO.B2O3 crystal powder can be prepared, for example, as follows. Namely, a material prepared to have a ratio of PbO:B2O3=4 mol:1 mol, is melted at 900xc2x0 C. for 1 hour, formed into flakes and then subjected to heat treatment at 440xc2x0 C. for 1 hour, whereupon the product is pulverized in a ball mill for a predetermined period of time to obtain a powder.
The Pb3O4 powder may, for example, be a commercially available product.
The sealing composition of the present invention comprises the above-described components. However, other components may also be incorporated within a range not to impair the purpose of the present invention i.e. in a total amount of up to 5 parts by weight per 100 parts by weight of the composition. As such other components, pigments such as black heat resistant pigments, may, for example, be mentioned.
The PbOxe2x80x94ZnOxe2x80x94B2O3xe2x80x94SiO2 type crystalline low melting point glass powder preferably comprises:
More preferably, it comprises:
The total content of BaO, CaO and SrO is preferably from 0.1 to 1.85 wt %, and the weight ratio of ZnO/PbO i.e. the weight ratio of the ZnO content to the PbO content, is preferably within a range of from 0.14 to 0.20.
The above compositions will be described below wherein wt % will be represented simply by %.
If the content of PbO is less than 71%, the softening point tends to be too high, whereby the fluidity deteriorates, the strength is likely to decrease and/or the air tightness of the sealing portion tends to deteriorate. It is preferably at least 71.5%, more preferably at least 74.5%. If it exceeds 84%, the softening point tends to be too low, and it tends to be difficult to make the temperature rising rate sufficiently large during exhaust. It is preferably at most 78%, more preferably at most 77%, particularly preferably at most 76%.
If the content of ZnO is less than 8%, the softening point tends to be too high, and crystallization tends to be difficult. It is preferably at least 9%, more preferably at least 10.5%, particularly preferably at least 11.5%, most preferably at least 12.1%. If it exceeds 16%, devitrification is likely to result during is the melting of glass. It is preferably at most 14.5%, more preferably at most 13.5%.
The weight ratio of ZnO/PbO i.e. the weight ratio of the ZnO content to the PbO content, is a parameter to satisfy both sealing in a short period of time and a high temperature rising rate during exhaust, and it is preferably within a range of from 0.14 to 0.20. If it is less than 0.14, the temperature rising rate during exhaust may not be made sufficiently large. It is more preferably at least 0.15. If it exceeds 0.20, the fluidity tends to decrease, and the strength tends to decrease. Further, crystallization tends to be difficult, and sealing in a short period of time tends to be difficult. More preferably, it is at most 0.18.
If the content of B2O3 is less than 7%, the softening point tends to be too high, and the fluidity tends to decrease. It is preferably at least 8%. If it exceeds 10%, the chemical durability tends to deteriorate. It is preferably at most 9.5%.
The content of SiO2 is an important parameter to make it possible to increase the temperature rising rate during exhaust. If it is less than 1%, the crystallization rate tends to be too large, and it tends to be difficult to make the temperature rising rate during exhaust sufficiently large. It is preferably at least 1.65%, more preferably at least 1.7%. In the vicinity of the upper limit within the allowable range of the content, the properties may not change so drastically as in the vicinity of the lower limit. However, if it exceeds 3%, the softening point tends to be too high, and the fluidity tends to decrease. Preferably, it is at most 2.5%.
BaO is not essential, but may be contained up to 3% in order to satisfy both sealing in a short period of time and a high temperature rising rate during exhaust. If it exceeds 3%, the crystallization rate tends to be too small, and sealing can not be carried out in a short period of time. It is more preferably at most 1.85%, particularly preferably at most 1.80%. When it is desired to further increase the temperature rising rate during exhaust, BaO is preferably incorporated in an amount of at least 0.1%, more preferably at least 0.6%.
CaO and SrO are not essential, but may be incorporated up to 3% each in order to improve the melting property of the glass. If the content exceeds 3%, the crystallization rate tends to be too small. More preferably, each content is at most 1.5%.
The total amount of BaO, CaO and SrO is preferably from 0.1 to 1.85% in order to improve the adhesion property. If it is less than 0.1%, the adhesion property may not be improved. Preferably, it is at least 0.5%. If the total amount exceeds 1.85%, the crystallization rate tends to be too small. More preferably, it is at most 1.8%.
Li2O, Na2O and K2O are not essential, but may be incorporated up to 3% each in order to improve the melting property of the glass. If either one exceeds 3%, the electrical insulating property is likely to deteriorate.
Al2O3 is not essential, but may be incorporated up to 5% in order to improve the chemical durability. If it exceeds 5%, the fluidity is likely to deteriorate.
Bi2O3 is not essential, but may be incorporated up to 10% in order to improve the fluidity. If it exceeds 10%, the crystallization tends to be poor.
The PbOxe2x80x94ZnOxe2x80x94B2O3xe2x80x94SiO2 type crystalline low melting point glass powder preferably comprises the above-described components. However, it may contain other components in a total amount of up to 3% within a range not to impair the purpose of the present invention. As such other components, a colorant such as Fe2O3 may, for example, be mentioned.
Further, it is preferred not to contain a halogen, especially fluorine, since a halogen, especially fluorine, is likely to gasify during the use of a color cathode ray tube and deteriorate the characteristics of an electron gun of the color cathode ray tube (emission slump phenomenon).
Now, a bulb of a color cathode ray tube sealed by the sealing composition of the present invention (hereinafter referred to simply as the bulb) will be described.
The bulb is produced by mixing the sealing composition of the present invention with a vehicle to obtain a paste, and coating this paste on the portions of the panel and the funnel to be sealed, followed by baking.
The average linear expansion coefficient (hereinafter referred to as the JIS expansion coefficient) within a range of from 0xc2x0 C. to 300xc2x0 C. of the panel and the funnel to be used for the bulb, is measured in accordance with the provisions of JIS R3102 and is preferably within a range of from 97xc3x9710xe2x88x927 to 99xc3x9710xe2x88x927/xc2x0 C.
Of the bulb, the strength (the sealing strength) of the portion having the panel and the funnel sealed, is measured by a four point flexural strength test method which will be described hereinafter. The measurements are carried out eight times, and the average value is taken as the sealing strength.
(1) A test piece of 60 mmxc3x9730 mmxc3x979 mm is taken from each of the panel and the funnel, and the surface of 60 mmxc3x979 mm is polished for mirror finishing. On the other hand, the sealing composition is kneaded with a behicle to obtain a paste. As the behicle, one having 1.2% of nitrocellulose dissolved in isoamyl acetate, is employed.
(2) The paste is coated on the mirror finished surface of the funnel test piece, and the panel test piece is placed thereon so that the mirror-finished surface thereof will face the paste-coated surface, and this assembly is fired in an electric furnace to bond the panel test piece and the funnel test piece.
(3) A test piece (5 mmxc3x9760 mmxc3x979 mm) having a width of 5 mm in a direction perpendicular to the bonded surface of the above bonded test pieces, is cut out and used as a strength test piece.
(4) Using the above strength test piece, four point flexural strength test (upper span: 20 mm, lower span: 50 mm, crosshead speed: 0.1 mm/min) is carried out, and the strength is calculated.
If the sealing strength is less than 500 kgf/cm2, it is difficult to reduce the weight of the bulb of a color cathode ray tube. Preferably, it is at least 520 kgf/cm2.
If the electric current (leak current) flowing between the exterior and the interior of the sealing portion of the panel and the funnel exceeds 30 nA when 30,000 V is applied across the exterior and interior of the sealing portion of the bulb, the sealing portion is likely to undergo dielectric breakdown.